1. Field of the Invention
The present invention relates to a fuel cell system to which a technique for detecting a failure due to leakage in a fuel gas channel is applied, and a method of detecting a failure in the fuel gas channel of the fuel cell system.
2. Description of the Related Art
For example, a polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode (fuel electrode) and a cathode (air electrode), and a polymer electrolyte membrane interposed between the electrodes. The electrolyte membrane is an ion exchange membrane. The membrane electrode assembly is sandwiched between a pair of separators. A fuel gas flow field is formed between the anode and one of the separators, and an oxygen-containing gas flow field is formed between the cathode and the other of the separators. In use, normally, a predetermined numbers of the membrane electrode assemblies and separators are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a hydrogen-containing gas is supplied to the fuel gas flow field. The fuel gas flows through the fuel gas flow field along the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the suitably humidified electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electrical energy.
Further, in the fuel cell, an oxygen-containing gas such as the air is supplied to the oxygen-containing gas flow field, and the oxygen-containing gas flows along the cathode for reaction. At the cathode, hydrogen ions from the anode combine with the electrons and oxygen to produce water. The water is retained at the anode due to the back diffusion from the cathode or high humidification of the fuel gas or the like.
If the water is excessively retained at the anode, water clogging may occur undesirably. Therefore, it is necessary to eliminate the water clogging.
Therefore, in the conventional technique, if impurities such as water are retained in the fuel channel of the fuel cell system, a purge process is carried out by a purge valve provided in the fuel channel. The purge valve is opened such that the fuel gas is discharged to the outside. By increasing the flow rate of the fuel gas, the impurities are blown away, and removed from the fuel channel.
Further, in this type of the fuel cell system, in a proposed technique, when operation of the fuel cell system is finished, an interruption valve in the fuel gas channel is closed such that the fuel gas pressurized at a certain pressure value or more is contained in the fuel gas channel including the fuel gas flow field in the fuel cell to stop power generation (see Japanese Laid-Open Patent Publication No. 2004-192919).
According to the technique disclosed in Japanese Laid-Open Patent Publication No. 2004-192919, as shown in FIG. 7, the decrease in the pressure value in the fuel gas channel is monitored from the containment time of the fuel gas as the end time to determine whether the decreasing rate curve of the pressure is within the normal range or within the abnormal range. If it is determined that the decreasing rate curve of the pressure is within the normal range at the time of starting up the fuel cell system next time, the start up operation of the fuel cell system is permitted. If it is determined that the decreasing rate curve of the pressure is within the abnormal range, the start up operation of the fuel cell system is prohibited.
In the technique, it is possible to detect the failure of the interruption valve provided in the fuel gas channel.
However, in the conventional technique, as shown in FIG. 7, even in the normal condition, as the contained fuel gas flows toward the cathode through the electrolyte membrane, the pressure decreasing rate decreases gradually from 100 [%] at the operation end time as the start point of the system stop.
Therefore, the required period for determining whether the condition is normal or abnormal, i.e., the period from the operation end time is long disadvantageously. Further, the degree of the decrease in the pressure of the fuel gas may change depending on various factors such as the containment pressure, the component, the shape, and the moisture of the electrolyte membrane, and the shape of the flow field of the separator. Therefore, for each fuel cell system, in FIG. 7, the threshold curve of the pressure decreasing rate denoted by the curve drawn by the dashed line in FIG. 7 needs to be stored in a memory, and the design is complicated. Thus, in the conventional technique, the determination process is complicated.
Further, in the conventional technique, the fuel gas and the oxygen-containing gas may be mixed at each of the electrodes of the fuel cell. Therefore, the performance of the fuel cell may be degraded undesirably.